Second harmonic generation technology in optical fibers
Second-order nonlinearity is of great significance in a range of applications, including precision frequency metrology, optical clocks, molecular imaging, and quantum information processing. Optical fibers, due to their high nonlinearity and compactness, are an ideal platform for studying nonlinear effects. However, the inversion symmetry of optical fibers makes it difficult to achieve second-order nonlinear effects, thereby hindering research on all-fiber second-order nonlinearity.
Currently, researchers have been able to directly achieve second harmonic generation (SHG) in the core or cladding of optical fibers. The phase-matching conditions for SHG in optical fibers are primarily satisfied through techniques such as self-organized phase matching, quasi-phase matching, and integrated material-assisted phase matching. However, SHG technology in optical fibers still faces various challenges, such as the separation of the fundamental wave (FW) source and the SHG medium, low average output power, and the need for complex preprocessing. Additionally, some studies have utilized Cherenkov radiation phase matching to achieve SHG; however, the second harmonic (SH) is not generated within the core but in the cladding, resulting in it being a leakage mode with rapid power decay and poor beam quality. How to directly generate high-beam-quality SH within the core using an all-fiber structure without preprocessing warrants further investigation.
Novel all-fiber random cavity structure directly generates high-beam-quality second harmonic in the fiber core
Recently, the Advanced Laser Technology Research Team at the Department of Precision Instruments, Tsinghua University, achieved all-fiber high-beam-quality SHG within the fiber core in a random fiber laser. In terms of gain, phase matching is primarily achieved through the periodic electric field induced by FW and SH, while utilizing the passive spatio-temporal gain modulation mechanism of the random laser and increasing the nonlinear gain length to enhance the second harmonic gain. In terms of feedback, a distributed feedback mechanism and point feedback devices were combined to form a random resonant cavity for SH. In the experiment, SHG required no preparation time or preprocessing. Due to the unique gain and feedback configuration, FW and SH were generated from the same random cavity, and SH was directly output from the fiber core, with an average output power of 10.06 mW. Additionally, this study proposes an innovative theoretical model that couples self-organized SHG theory with the generalized nonlinear Schrödinger equation, enabling synchronized simulation of the spectral evolution of SH and FW. This structure fully leverages the advantages of optical fibers, achieving all-fiber high-beam-quality SHG within the fiber core, with potential applications in environmental sensing, fiber optic communication, and optical frequency combs.
The findings were published in the March 2025 issue of High Power Laser Science and Engineering (Yousi Yang, Dan Li, Pei Li, Guohao Fu, Tiancheng Qi, Yijie Zhang, Ping Yan, Mali Gong, Qirong Xiao, "Second-harmonic generation within a random fiber laser," High Power Laser Sci. Eng. 13, 03000e41 (2025)).
The structure of the random fiber laser is shown in Figure 1. The output light from the laser diode pump source is coupled into the fiber cladding via a combiner and injected into the ytterbium-doped fiber, where the pump light is converted into the fundamental wave. The core light is then injected into a 1-kilometer-long communication fiber, with its output end angled to avoid Fresnel reflection coupling into the core. In the reverse output direction, the signal fiber of the combiner is connected to a high-reflectivity grating, forming a semi-open ytterbium-doped random laser cavity. The other end of the grating is connected to a single arm of a 2×1 coupler, while the other two arms of the coupler are directly fused to form a fiber ring mirror.
Figure 1(a) Experimental setup diagram (LD Pump: laser diode pump source; HR FBG: high-reflectivity fiber Bragg grating; YDF: ytterbium-doped fiber; GDF: germanium-doped fiber; CPS: cladding power stripper); (b) Second harmonic gain and feedback principle; (c) Second harmonic laser spot after cladding stripper processing; (d) Visible light in the fiber during pumping
The experiment employs a cascaded double random cavity structure, with the inner cavity formed by a high-reflection fiber Bragg grating as the ytterbium-doped gain cavity, and the outer cavity achieving broadband feedback via a ring mirror. The output spectrum and power are shown in Figure 2. When the pump power approaches the threshold, a main peak appears in the spectrum accompanied by random noise peaks, originating from the self-modulation Q effect due to Rayleigh scattering and stimulated Brillouin scattering. At this point, the time-domain strong pulse excites the 535 nm second harmonic. When the pump power exceeds the cascaded Raman threshold, the spectrum broadens into a supercontinuum (680–2116 nm). In the near-infrared region, the fundamental wave and higher-order Stokes light participate in SHG, resulting in a main peak at 592 nm in the SH band. This is because the orange light has low loss and the 1184 nm Raman light power is sufficient.
Figure 2 Output spectra at FW powers of (a) 0.65 W, (b) 13.6 W, and (c) 20.88 W; (d) comparison of SH spectra; (e) SH output power
Furthermore, an optical filter device was used to remove stray light above 680 nm to study the SH time-domain characteristics. Figure 3(a) shows that the SH waveforms exhibit significant intensity fluctuations at different fundamental powers, with some pulses far exceeding the average intensity, indicating the possible presence of optical rogue waves. The statistical histogram (Figure 3(b)) reveals an L-shaped distribution characteristic, with the gray area representing the noise background and the dashed line marking the peak amplitude twice the significant wave height. The pulse width of the rogue waves in Figure 3(c) is limited by the detector bandwidth (which may be narrower in practice). Due to differences in phase-matching positions, SH at different wavelengths exhibit subtle adjustments in output timing, with secondary peaks appearing alongside the main peak, corresponding to SH components of low-intensity FW.
Figure 3 Time-domain characteristics of SH at different powers. a) Wide-time-range waveform. b) Time-domain intensity distribution histogram. c) Single-pulse waveform measurement results
This study proposes an all-fiber core SHG method that inherently possesses stronger pump injection capability without requiring special processing. Additionally, this structure can simultaneously generate FW and SH, offering a highly integrated design. Further research includes designing feedback responses for SH wavelength tuning, increasing pump power to meet higher-power applications, and conducting in-depth studies on the odd wave phenomenon.
